More than 70 maternity units rely on a screening test which misses one in three cases of the disability – despite the existence of far more accurate technology.

Other hospitals are using methods which falsely identify too many pregnant women as "high risk," putting thousands under needless pressure to undergo invasive procedures which increase their risk of miscarriage.

The findings reveal a postcode lottery with the most accurate results recorded in hospitals in London, while the tests with the lowest detection rates are in most common use at hospitals in the North and Midlands.

Experts said few women realised their choice of hospital could affect the accuracy of their screening, with some units still using methods they described as "absolutely substandard".

Around 750,000 pregnant women are offered NHS screening every year in an attempt to detect those babies most likely to have Down's syndrome, and 60 per cent accept the offer.

They undergo an initial screening test to determine whether they are at "high risk" of carrying a child with Down's.

Those found to be at high risk are offered an invasive procedure, such as an amniocentesis or chorionic villus sampling (CVS), which is the only way to make a definite diagnosis.

However, these procedures, which involve a needle inserted into the placenta or uterus, carry around a one per cent risk of miscarriage – often higher than the chance that the baby will have Down's.

In cases where a positive diagnosis is reached, 92 per cent of women choose to terminate the pregnancy.

Figures obtained by this newspaper show that one in three of England's 205 maternity units is still using a screening method which detects just 65 per cent of cases, despite the fact other hospitals have achieved detection rates of 90 per cent.

Meanwhile, maternity units in Kent and Sussex are using screening methods which categorise as many as one in five women as being at "high risk" of carrying a child with Down's.

By contrast, the best screening programmes, in use by 15 NHS hospitals, identify about 90 per cent of Down's cases, after narrowing down to only two per cent of women via screening.

From April, all maternity units in England are supposed to meet a "75/3" standard – meaning the tests detect 75 per cent of cases of Down's syndrome, from 3 per cent of women who "screen positive" as high risk.

However, Pat Ward, director of the NHS Fetal Anomaly Screening Programme, said that while some hospitals are achieving better results, the target will be missed.

The programme says the best screening to identify the risk of Down's is a "combined" test, which brings together information from an ultrasound scan, measuring the spinal cord at the back of the baby's neck, and from blood tests.

It detects between 75 per cent and 90 per cent of cases of the disability.

It also supports the use of a "quad test" which measures four hormone levels, with similar results, despite the fact the test cannot be carried out until a woman is four to five months pregnant.

Records from this month show that 99 hospitals are now using the combined test, including 15 with 90 per cent detection rates. Another 27 are using the quad test, while four use an integrated test, which has similar results.

But 71 hospitals, mainly from the North and the West Midlands, are still using an outdated "triple second" test.

The blood test, which cannot be carried out until a woman is four months pregnant, detects just 65 per cent to 70 per cent of cases of Down's, after identifying six per cent of women as "high risk".

Meanwhile, three hospitals are still offering women tests which were supposed to be phased out three years ago, the investigation found.

Maternity units at Eastbourne General Hospital and the Conquest Hospital in Sussex and Medway Maritime Hospital in Kent only offer pregnant woman an ultrasound scan, which identifies as many as 20 per cent of women as being at high risk.

The high "positive screen rate" of the ultrasound nuchal fold scan – when used without blood tests – means that far more women end up advised to have high-risk invasive tests.

Jane Fisher, director of charity Antenatal Results and Choices, said that although all pregnant women in England are allowed a choice of hospital, few were aware of crucial differences between the tests offered.

She said: "The quality of the tests is very patchy, especially outside London and the south east, and the use of the nuchal fold test on its own is absolutely substandard.

"A small number of women do a lot of detective work, and some will go privately if the tests offered locally aren't of a high standard, but the vast majority are not aware of the differences."

Many women called the charity's helpline after being given a "high risk" categorisation which left them agonising about whether or not to have invasive tests.

"A high proportion of those who come to us do so because they have been told the risk of Down's is similar to the risk of miscarriage, and that leaves them with a terrible dilemma," she said.

Mrs Ward said the NHS was working hard to keep up with major advances in screening in the last decade.

She said: "Britain is a world leader in screening for Down's syndrome, but we want to keep improving, both in terms of the percentage of cases we detect, and in reducing the number of women who go through a diagnostic test which carries a miscarriage risk."

Mrs Ward said she expected 85 per cent of hospital trusts to hit the 75/3 standard by this April. Hospitals have been given until 2011 to meet the 90/2 target.

The chance of having a child with Down's syndrome increases dramatically with age. While a 20 year old has a 1 in 1,500 chance of carrying a child with Down's, by the age of 40, the chance is 1 in 100.

The number of diagnoses has risen steeply over the past two decades as women have postponed having families, but fewer babies with the condition are being born as increasing numbers of women opt for termination, after the disability has been identified.

Around 750 babies a year are born with Down's syndrome in the UK. More than 1,000 pregnancies are terminated annually after the disability is identified.

Sarah James, from South London, was 41, and a mother of four when she became pregnant.

At 12 weeks, a nuchal fold scan identified her child as having a "high risk" of Down's, but making a diagnosis would require her to undergo a CVS test with a one per cent risk of miscarriage.

Mrs James and her husband, both practising Roman Catholics, deliberated. She said: "My youngest child was only four years old, and we already had four children, so the idea of having a baby with Down's was very daunting.

"We really didn't think we would manage with the pressures, so I went ahead with the CVS."

The results provided reassurance for Mrs James and her husband; their child did not have Down's syndrome.

"I began to relax," said Mrs James, a childminder. "I told the older children that I was expecting, and started to buy baby clothes."

Weeks later, she began to bleed heavily. Obstetricians informed her that her child had died several weeks previously. No one could say whether the miscarriage had been triggered by the invasive procedure.

Mrs James said: "I felt very guilty, and confused. I had been buying baby clothes for a child who was already dead.

"No one could say if the CVS had caused the miscarriage, I was 41, and it could have happened anyway, but I struggled with the guilt for a long time".

Charlotte Gardner, 37, a teacher for the National Childbirth Trust, had a nuchal fold scan, to detect the risk of Down's syndrome.

She was reassured to be told that her unborn child was low risk.

But when her baby Gemma was born, it was clear from her appearance that in fact she suffered from the disability.

Charlotte, and her husband Mark struggled to cope with the news, as they got to know their new baby, their second child.

Two years on, Mrs Gardner says it is impossible to consider the idea that she might have terminated the pregnancy. She describes Gemma as a sociable, outgoing two year old, who has enriched the lives of all around her.

However, Mrs Gardner believes screening tests which alert expectant parents to the statistical risk that their child has Down's syndrome are no use.

"I think the screening gives people false fears and false hopes. If you want to know if your child has Down's, the only way to know is to have the amnio or CVS.

"Even if statistics tell you you've just a 1 in 1,000 risk of having a child with Down's, if that turns out to be your child, the odds don't make any difference."

For more information on Down Syndrome screening, Antenatal Results and Choices can be contacted on their helpline 0207 631 0285 or at their website www.arc-uk/org